Reference may be made to the following U.S. Pat. Nos. of interest: 4,542,649; 4,660,416; 4,690,003; 4,693,113; 4,712,071; 4,735,101.
Fluid control valves are used in a wide variety of applications such as oil and gas pipelines and processing lines, as well as to control water and other fluids in nuclear power generating stations. In such critical applications, substantial maintenance is required, both of the periodic preventative maintenance type as well as to repair valve breakdowns in order to assure that the control valve performs properly, thereby reducing losses associated with process fluid leakage and trim damage. It has been desired therefor to provide an easy and readily available technique for diagnostically checking such fluid control valves already installed in the system so that operational problems can be detected before they become the source of fluid process losses or, in the worse case, cause unscheduled shutdowns of the processing system.
A motor operated valve analysis and testing system is described in the above-mentioned patents for fluid control valves operated by a very specific type of electric actuator including a motor moving the valve stem through a worm gear, and a cluster of compression springs termed a "spring pack" for reacting to valve stem thrust. Spring pack movement is detected to provide an indirect indication of stem load. However this first requires that a calibration be done where the stem load is measured using a load cell and the readings correlated to spring pack movement. The load cell is then removed and the spring pack reading used to measure stem load during operation. Motor current is also measured and is correlated to stem load to give an indication of the overall condition of the motor. This prior art system is time consuming and cumbersome in requiring the initial calibrations as well as requiring a significant amount of time to install and remove the load cell on each actuator to be tested.
Furthermore, many fluid control valves, such as pneumatically actuated control valves do not have a motor or spring pack so that the valve measurements described earlier in connection with electrically actuated valves does not apply to such pneumatic units. Accordingly, it is desired to provide an improved technique for diagnostically testing fluid control valves, such as pneumatically operated valves in a fast and efficient manner.